Nitinol and similar shape memory materials (SMM) have unique material properties, which allow them to be pre-formed in a desired “memorized” shape and return to that shape after any deformation above a material specific phase-change temperature. When a heat source is removed and the temperature drops below the phase-change temperature, the shape memory material retains a deformable state. The temperature range for controlling the shape may be varied based on the composition of the SMM and the techniques used to process and form the SMM. Due to the ability to control the shape of such a flexible, yet strong material has led to a variety of applicable uses for SMM.
However, the traditional methods for constructing and controlling a SMM, or actuators that are embedded with SMMs, has been limited for several reasons. SMMs are slow to reach the disparate temperatures required for transition without outside intervention. The SMM typically requires a cooling source to reduce the temperature of the material after being driven to the opposite phase to allow for transition back to the deformable state. To obtain the required temperature change, the materials are typically constructed with individual shape memory elements connected at their ends in much the same way a traditional wire wound resistor is constructed. However, these constructions formed with individual SMM elements have very limited geometries and require complex multipart mountings. Furthermore, the constant flexing of the SMM causes the mechanical connections between individual SMM elements to fatigue and rapidly fail overtime, which reduce the overall durability and longevity of these actuators, greatly increasing replacement and maintenance costs.
In addition, traditional methods to control multiple mechanical axis and multiple degrees of freedom of the actuators with SMMs have resorted to using individual conductors to heat each individual SMM element. The more SMM elements to drive the actuator, the more wiring that is required, which results in a larger size actuator, higher costs, reduced durability, and can lead to undesired thermal management design considerations. As such, the current actuators and designs have resulted in poor performance and their control has been limited to only one or two degrees of freedom. Having only one to two degrees of freedom of control greatly limits the use of SMM for a variety of applications.
The limited control of a unitary, monolithic structure of SMM to form a shape memory actuator (SMA) has also been described in the literature, such as the SMA described in U.S. Pat. No. 4,551,975. The disclosed SMA includes a plurality of separated conductors interfaced with a surface of SMM. A controller forms circuits between two or more specific conductors to control a path of current in SMM situated between those specific conductors to heat and activate those sections of SMM. However, the configuration of this SMA has limited control resolution, among other drawbacks. In general, a higher degree and resolution of control of particular sections of a unitary, monolithic SMA requires additional conductors and conduction points to heat specific sections of the SMM. As the density of conductors increase, the signal pathways for control (i.e., the controller leads directing the current) also need to increase, where the number and availability of signal pathways may become a limiting factor affecting the potential control resolution of the SMM.
Thus, there is a need in the art for new structural designs of shape memory actuators and methods of control thereof to improve the overall versatility, agility, and control resolution of SMAs. There is a further need to control an actuator or SMM in multiple axes and in multiple degrees of freedom to greatly expand their applicable uses.